Add missing GRPC header

[pleshakov]

F5 telemetry service requires the presence of the X-F5-OTEL set to GRPC.

The exporter doesn't set this header by default.

Workaround - set it using Options when creating Exporter:

                . . .
		options := []otlptracegrpc.Option{
			otlptracegrpc.WithEndpoint(cfg.ProductTelemetryConfig.Endpoint),
			otlptracegrpc.WithHeaders(map[string]string{
				"X-F5-OTEL": "GRPC",
			}),
		}
		if cfg.ProductTelemetryConfig.Insecure {
			options = append(options, otlptracegrpc.WithInsecure())
		}

		var err error
		exporter, err = tel.NewExporter(
			tel.ExporterConfig{
				SpanProvider: tel.CreateOTLPSpanProvider(options...),
			},
			tel.WithGlobalOTelLogger(logger),
			tel.WithGlobalOTelErrorHandler(errorHandler),
		)

However, it should be done by default by the exporter library